Slow slip and the transition from fast to slow fronts in the rupture of frictional interfaces

The failure of the population of micro-junctions forming the frictional interface between two solids is central to fields ranging from biomechanics to seismology. This failure is mediated by the propagation along the interface of various types of rupture fronts, covering a wide range of velocities. Among them are so-called slow fronts, which are recently discovered fronts much slower than the materials' sound speeds. Despite intense modelling activity, the mechanisms underlying slow fronts remain elusive. Here, we introduce a multi-scale model capable of reproducing both the transition from fast to slow fronts in a single rupture event and the short-time slip dynamics observed in recent experiments. We identify slow slip immediately following the arrest of a fast front as a phenomenon sufficient for the front to propagate further at a much slower pace. Whether slow fronts are actually observed is controlled both by the interfacial stresses and by the width of the local distribution of forces among micro-junctions. Our results show that slow fronts are qualitatively different from faster fronts. Since the transition from fast to slow fronts is potentially as generic as slow slip, we anticipate that it might occur in the wide range of systems in which slow slip has been reported, including seismic faults.Comment: 35 pages, 5 primary figures, 6 supporting figures. Post-print version with improvements from review process include

Using the UM dynamical cores to reproduce idealised 3D flows

We demonstrate that both the current (New Dynamics), and next generation (ENDGame) dynamical cores of the UK Met Office global circulation model, the UM, reproduce consistently, the long-term, large-scale flows found in several published idealised tests. The cases presented are the Held-Suarez test, a simplified model of Earth (including a stratosphere), and a hypothetical tidally locked Earth. Furthermore, we show that using simplifications to the dynamical equations, which are expected to be justified for the physical domains and flow regimes we have studied, and which are supported by the ENDGame dynamical core, also produces matching long-term, large-scale flows. Finally, we present evidence for differences in the detail of the planetary flows and circulations resulting from improvements in the ENDGame formulation over New Dynamics.Comment: 34 Pages, 23 Figures. Accepted for publication in Geoscientific Model Development (pre-proof version

The rainbow as a student project involving numerical calculations

It is a challenge to find interesting and realistic projects where numerical methods can be used to enhance student understanding of physical phenomena. We present such a project in which a group of students used numerical methods to analyze the physics of the rainbow. The project is suitable for students in an undergraduate physics course on the basic principles of geometrical optics. The central part of this paper is written by a group of students, and the introduction and discussion are written by their teacher. In this way both the students' and teacher's perspectives on using numerical methods are presented

Minimal model for slow, sub-Rayleigh, supershear, and unsteady rupture propagation along homogeneously loaded frictional interfaces

International audienceIn nature and experiments, a large variety of rupture speeds and front modes along frictional interfaces are observed. Here, we introduce a minimal model for the rupture of homogeneously loaded interfaces with velocity strengthening dynamic friction, containing only two dimensionless parameters; τ, which governs the prestress, and ᾱ which is set by the dynamic viscosity. This model contains a large variety of front types, including slow fronts, sub-Rayleigh fronts, super-shear fronts, slip pulses, cracks, arresting fronts and fronts that alternate between arresting and propagating phases. Our results indicate that this wide range of front types is an inherent property of frictional systems with velocity strengthening branches

The need for laboratory work to aid in the understanding of exoplanetary atmospheres

Advancements in our understanding of exoplanetary atmospheres, from massive gas giants down to rocky worlds, depend on the constructive challenges between observations and models. We are now on a clear trajectory for improvements in exoplanet observations that will revolutionize our ability to characterize the atmospheric structure, composition, and circulation of these worlds. These improvements stem from significant investments in new missions and facilities, such as JWST and the several planned ground-based extremely large telescopes. However, while exoplanet science currently has a wide range of sophisticated models that can be applied to the tide of forthcoming observations, the trajectory for preparing these models for the upcoming observational challenges is unclear. Thus, our ability to maximize the insights gained from the next generation of observatories is not certain. In many cases, uncertainties in a path towards model advancement stems from insufficiencies in the laboratory data that serve as critical inputs to atmospheric physical and chemical tools. We outline a number of areas where laboratory or ab initio investigations could fill critical gaps in our ability to model exoplanet atmospheric opacities, clouds, and chemistry. Specifically highlighted are needs for: (1) molecular opacity linelists with parameters for a diversity of broadening gases, (2) extended databases for collision-induced absorption and dimer opacities, (3) high spectral resolution opacity data for relevant molecular species, (4) laboratory studies of haze and condensate formation and optical properties, (5) significantly expanded databases of chemical reaction rates, and (6) measurements of gas photo-absorption cross sections at high temperatures. We hope that by meeting these needs, we can make the next two decades of exoplanet science as productive and insightful as the previous two decades.Publisher PD

1D model of precursors to frictional stick-slip motion allowing for robust comparison with experiments

We study the dynamic behaviour of 1D spring-block models of friction when the external loading is applied from a side, and not on all blocks like in the classical Burridge-Knopoff-like models. Such a change in the loading yields specific difficulties, both from numerical and physical viewpoints. To address some of these difficulties and clarify the precise role of a series of model parameters, we start with the minimalistic model by Maegawa et al. (Tribol. Lett. 38, 313, 2010) which was proposed to reproduce their experiments about precursors to frictional sliding in the stick-slip regime. By successively adding (i) an internal viscosity, (ii) an interfacial stiffness and (iii) an initial tangential force distribution at the interface, we manage to (i) avoid the model's unphysical stress fluctuations, (ii) avoid its unphysical dependence on the spatial resolution and (iii) improve its agreement with the experimental results, respectively. Based on the behaviour of this improved 1D model, we develop an analytical prediction for the length of precursors as a function of the applied tangential load. We also discuss the relationship between the microscopic and macroscopic friction coefficients in the model.Comment: 13 pages, 14 figures, accepted in Tribology Letter

Modelling the onset of dynamic friction : A study of rupture velocities

The onset of dynamic dry friction between two blocks of PMMA has recently been studied experimentally. Technological advances enable the study of the onset of sliding at high temporal resolutions, resulting in new insights regarding how slip occurs at a local scale along the interface. In this thesis, spring-block models are used to study the onset of dynamic dry friction. These models have been used to study earthquakes for a long time, and have later been adopted to study friction on a laboratory scale. The agreement with the mentioned experimental results has, however, up until now been rather poor. In these models, which are fully resolved in time, the local friction law has to be imposed. Using a local Amontons--Coulomb friction law, the one-dimensional model shows qualitative agreement with the experimental results. A quantitative comparison, however, reveals serious discrepancies. Studies reveal the importance of dimensionality to the kinetics, i.e. the states where slip nucleates and arrests, of the system. An example is the length of precursors as a function of the applied tangential load, which appears to be improved by including two-dimensional effects in the one-dimensional model. Both dimensionality and the local friction law are seen to be crucial for the dynamics of the system, e.g. micro-slip front propagation. In the one-dimensional model, a specific relationship between the rupture velocity, the initial shear to normal stress ratio and the local friction coefficients is derived. This reveals the importance of the interfacial strength to the rupture velocities in addition to the stress ratio which has been suggested in the experimental papers. A (2 + 0)D model, where both horizontal dimensions of the system are included, is also studied. It is shown that details of how the driving force is applied are crucial. As an illustration, a precursor has in a (2 + 0)D system both a length and a size. The development of these two quantities as a function of the applied tangential load can significantly deviate from each other depending on how the driving force is applied. The lack of experimental results studying these effects, however, makes the analysis difficult